Method and apparatus for locating items

ABSTRACT

A convenient handheld locator is provided for locating an item in an urban environment in which the locator is programmed to search for and locate specific items, with the detected item being displayed on the locator as to its identity or name, also displaying where the item is relative to the locator, as to position and range.

RELATED CASES

This is a continuation of co-pending U.S. patent application Ser. No.13/187,768 filed Jul. 21, 2011.

FIELD OF INVENTION

This invention relates to a method and apparatus for locating misplaceditems utilizing RFID technology, more particularly to a handheld locatordevice that projects energy in a narrow beam and receives energyreturned from an illuminated and previously encoded RFID device in whichthe identity of the detected item is displayed along with its directionand distance from the handheld device, all based on a previoussynchronization operation of the locator with a computer-generated itemlisting.

BACKGROUND OF THE INVENTION

Oftentimes it is useful for anyone who lives in a house, apartment,tent, or other environment or who works in an office, school, warehouse,factory, farm, zoo, underground mine or other enclosed space to findmisplaced items. One might want to find books, keys, tools, papers,various equipment, magazines or anything else that can be lost in abuilding or crowded area.

As will be apparent, tools can fall from a workbench. Books can bemislaid on a couch behind pillows or in another room. Items such asremote clickers, eyeglasses, and even such items as small as a screw orbolt can oftentimes get lost and a great deal of time is spent trying tolocate these items.

In the past, EPC-standard RFID tags have been used to identify items,with the RFID tags encoded with appropriate identification informationsuch as a barcode. Additionally, golf balls can be found by a so-calledRadar Golf Ball Finder, with the technology used described in UnitedStates Patent Application Publication Number U.S. 2006/0128503. In thisPatent Publication a golf ball locator receives RF signals back from anRF circuit within the golf ball. The range to the golf ball isdetermined by the received signal strength using a received signalstrength indicator, or RSSI, with a threshold being set to indicate golfball detection. In one embodiment of the golf ball locator, the carrierfrom the locator is modulated to provide a spread spectrum binary phaseshift keyed, BPSK, modulated signal, where the modulation includes apseudo-random binary sequence, also known as a pseudo noise or PN code.Transmitted pulses are used to locate the golf ball which has an RFcircuit that returns a harmonic of the transmitted signal back to thelocator.

Additionally, RFID tags have been utilized to locate animals in which anRFID tag is attached to the animal.

One problem with all of the above RFID applications is that they operateout in the open. There are no multi-path or standing wave problems inwhich signals bounce off metal in the urban environment and buildingstructures which makes indoor reading of RFID tags difficult. Theseapplications are quite different from RFID readers which are adjacenttagged articles and which operate at low power.

The problem with EPC-standard RFID tags is that they are intentionallylimited in range due to limits on the transmit power established by theFederal Communications Commission. What typically happens with an RFIDtag is that energy from a source is used to power the tag, with the tagthen re-radiating its code back to the source, whereby the tag isidentified. The Federal Communications Commission intentionally limitsthe amount of power that can be utilized to power these tags so that thetag must be relatively close to the interrogating head in order for theRFID tag to work.

With respect to radar golf, the radar golf system discussed above doesnot distinguish which golf ball is detected and is range-limited toapproximately 100 feet. As to animal tags, the tags have batteries andare difficult to reconfigure.

SUMMARY OF THE INVENTION

The subject invention involves a convenient handheld locator forlocating an item in an urban environment in which the locator isprogrammed to search for and locate specific items. This provides aconvenient method and apparatus for locating a specified item, with thedetected item being displayed on the locator as to its identity or name.Also, where the item is relative to the locator is displayed, both as toposition and range.

The subject locator is useable indoors and out, and in one embodimentuses a long range batteryless tag with a variable power directionalinterrogator or locator similar to a flashlight. Software is provided tomake the locator work inside where signals bounce off metal in walls andfurniture. Moreover, range is increased by transmitting the unique codeto which the tag responds which significantly increases thesignal-to-noise ratio and thus increases range and the robustness of thesystem.

It is a feature of the subject invention that the locator is providedwith a software interface such that items tagged with individual codingdevices such as surface acoustic wave devices may be identified by thereturned radiation from the tag, with the item being identifiedon-screen by its name. The locator ascertains by the returned signalwhich item has been found, and upon detection its identity is displayedby a simple word such as “magazine”. The identity of the tag ispreviously uploaded to the locator from an item list in a computer.Thus, items can be prelabeled such as for instance “eyeglasses” or“book”, with the locator having a display that displays the nameassociated with the tag code for the item detected. In one embodimentthis association is provided through a USB or wireless link in ahot-synching operation.

The tags can be placed on books, personal articles such as passports,wallets, phones and keys, tools, magazines, pets, equipment, clothing oranything likely to be misplaced or lost. In one embodiment, the locatoroperates as a reader that works like a flashlight. It is directional sothat one can point it in different directions and find lost articleswhen they are detected. A ranging algorithm estimates how close thereader is to the lost article and in another embodiment the output powerof the transmitter can be varied so that one can for instance choose anISM band where increased power is permissible.

The tags on the other hand are simple, batteryless and thereforeinexpensive. The tags can be attached to various articles, with manyform factors possible. One form factor resembles adhesive tape or theDymo plastic embossed tape that was prevalent in the 1970's.

In one embodiment, each tape tag contains a dipole antenna and a circuitutilizing a diode to both collect the energy from incoming signal topassively power the tag, and to produce a harmonic output modulated foridentification purposes. Alternatively, a diodeless tag using a SAWdevice between input and output antennas may be used. When using eithera diode circuit or a diodeless tag, the SAW device may be used to encodethe tag with its unique impulse response, thus to provide an ID for thetag. Note that tags having different SAW devices are affixed todifferent items to uniquely identify them. To increase the robustnessand range of the subject system, in one embodiment the transmittedenergy carries the code of the sought-after item. A keyboard on thelocator is used to specify the item sought by simply typing in its name.This, in turn, programs the transmitter in the locator with theappropriate coding. When the item is located, the name of the item foundpops up on the display of the locator. In one embodiment, the directionand distance is presented.

As part of the subject invention, a computer such as a PC is connectedto the interrogator through a USB or wireless link, with the processbeing similar to synching an iPod to a PC. In this way the useridentifies items by familiar names that the interrogator is to detect.

In order to improve range several techniques are used. First, a highlydirectional antenna is used. This can be a narrow band patch antenna oran array of small dipoles such as are available for cell phones. Also,helix and yagi type antennas may be used. Secondly, cross correlationtechniques provide a cross ambiguity function, correcting for frequencyand range. Thirdly, frequency sweeping techniques are used inconjunction with a pseudo noise (PN) code to increase thesignal-to-noise ratio and thus increase range. Fourthly, a frequencyestimation system is used to correct for oscillator shift. Fifthly, aunique ranging algorithm is used. Sixth, an optional battery-poweredbiasing circuit for the diode detector extends range by a factor of 50%over unbiased diode tags. Finally, coding the transmitted signal withthe code carried by the sought-after tag and correlating the transmittedand received code increases range.

Note that indoor propagation is nothing like outdoor propagation.Indoors, one obtains reflections from metallic objects; and one has totake into account standing wave effects. One cannot assume plane waveradiation. Instead, urban location workshops have established that theoperator must move between “hot spots” where received signal strength ishigh. Therefore, the subject processing technique includes in oneembodiment correlating the received signal with the anticipated signalin order to estimate signal amplitude. To improve robustness and asdiscussed above, in one embodiment the radiated signal from the locatoris modulated with the ID code of the tag. Also, a small amount offrequency correction is provided through cross ambiguity functionprocessing.

In order to find an object, the operator must move toward areas of highsignal strength. The directional antenna provides a cue for locatingthese areas. It is assumed that signal strength is maximum when theinterrogator is pointed in the direction of a hot spot. However, it ispossible that the direction is a spurious reflection. Walking toward thespurious reflection still lets one find an object because the operatorends up tracing through the reflection and on toward the actual locationof the object.

In summary, the interrogator has a directional antenna and means fordetecting returns from tagged objects, as well as optionally estimatingreceived signal strength. By walking through areas of high signalstrength the operator traces a path toward the actual location of theobject. When the object is in the direct field of view, signalpropagation becomes less complex and the object is located either usingmonopulse techniques or through simple bearing estimation as is doneoutdoors using directional antennas.

In operation, the customer points the interrogator in a generaldirection to look for the lost article. The display shows the name ofany article detected along with a measurement of progress such as“article detected”, “look left for the article”, “the article is atrange x”.

The subject reader being directional increases range and can projectlarge amounts of power in a given direction to find lost articles, eventhrough walls which attenuate tag signals. At the same time, the powerlevel is controllable both for safety and to reduce interference.

The system is provided with digital processing inside the interrogatorto substantially increase range. In one embodiment, a variation on crossambiguity function processing techniques enables the reader to detectreturned signals falling well below the noise floor and utilizescorrelation techniques and frequency estimation methods to increaserobustness.

In another embodiment, rather than utilizing a batteryless tag, the tagis provided with a thin film lithium ion battery. It is been found thatbiasing the diode in an RFID tag with a battery increases the range by50%. This means that articles can be more readily found behind walls,behind blocking objects such as a pillow on a couch, and can be made tooperate at much longer ranges than those associated with the golf balldetection technology discussed above. In one embodiment, ranges inexcess of one mile are possible.

While a number of RFID tag technologies are useable in the subjectinvention, back scatter using a SAW delay line and BPSK Barker codingnot only increases range for passive batteryless tags, but also due tothe different impulse response of a SAW device uniquely identifies orcodes the item. An RFID kit can involve as many as ten tags, each withits own specially coded SAW device.

Other tags usable in the subject invention include passive AM modulatedbackscatter tags, and multi-resonant load on tags with a chirped/sweptinterrogator. Tag technologies include remodulated backscatter withfrequency offset from phase shifters, remodulated backscatter withfrequency offset from a local oscillator and mixer, re-modulatedbackscatter using harmonic generation involving both active and passivedevices, use of a “rectenna” and a so-called smart chip, a dispersiveantenna for chirp charging of “rectenna” type tags and a passivetransceiver, commercial transceivers using RFID commercial off-the-shelfparts and time expired beacon technology.

It has been found that an amplified SAW tag gives the most range andbasically involves amplified backscatter cross sections with modulationand a one microsecond time delay to differentiate between otherreflections. Note that SAW delay devices avoid the clutter by using timedelays. These SAW delay lines have the most range of all the passivetags.

Note also that remodulated tags avoid clutter by either phase modulationto offset the carrier frequency, or by mixing with a local oscillator,or by using harmonic generation. All of these techniques reducebackground and ease the requirement on dynamic range.

By way of further background, in the commercial world there are at leasttwo passive technologies. First, so called “rectennas” are used toharvest power from the interrogating signal. For example, the commonentry badge reader uses a rectenna which charges up a capacitor anddrives internally-carried memory and an oscillator in a smart chipconfiguration. The main limit for this technology is the required −30dBm charging power. Second, and behind the rectenna technology, are SAWdispersive delay lines at the feed point of the tag antenna. The“barcode” information is contained in the SAW device and is provided byspecifically placed reflectors in the SAW path. These reflectorsindicate one or zero at specific delays along the path.

In summary, passive batteryless tags are used with modulation techniquesto indicate the identity of the tag. Upon interrogation by the reader,the tag powers itself and emits an RF signal which is transmittedomni-directionally. This emitted signal is transmitted back to thereader with the encoded information. Upon receipt, the reader displaysthe selected name for the detected tag, thus quickly identifying whathas been found. The displayed tag name is uploaded to the reader from acomputer which stores a list of tag IDs and the user-defined name forthe particular tag.

The reader is provided with a relatively narrow beam antenna such asprovided by a yagi or helix, with the extended range of the systeminvolving frequency sweeping and pseudo noise coding to whichcorrelation techniques are applied. Further, range extension isaccomplished by transmitting the code for the tag sought and usingcorrelation techniques. A 50% range increase is also possible when usingbattery-powered diode biasing. Further, range to an item is detectedusing a unique range algorithm.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the subject invention will be betterunderstood in conjunction with the detailed description in connectionwith the drawings of which:

FIG. 1 is a diagrammatic illustration of the subject locator in whichactuation keys code the item to be found and in which a narrow beam isprojected in an area to which the item is thought to be located, withmodulated backscatter twice the frequency of the projected signal beingdetected by the locator and the item detected displayed;

FIG. 2 is a diagrammatic illustration of one embodiment of the subjectlocator showing the key pad for specifying the item to be found, a FINDbutton, and a display which includes an indication of the item foundalong with its location and direction;

FIG. 3 is a diagrammatic representation of the subject invention inwhich a variable power transmitter transmits a sweep of energy throughits directional antenna, with the energy intercepted by an RFID taghaving a diode, in which the backscatter from the tag is coded andreceived, wherein a processor applies a cross ambiguity function andranging algorithm to output the range to and the ID of the tag found;

FIG. 4 is a block diagram showing the transmission of a codedinterrogation signal towards a tag, and a diode tag having a SAW devicepositioned between the diode and the transmit antenna to code the tagdue to the different impulse response of the SAW device;

FIG. 5 is a diagrammatic illustration of one embodiment of the subjectlocator showing a keyboard with which to enter the item to be found, aFIND button, a directional antenna, and a display which displays theidentity of the found item and its location;

FIG. 6 is a diagrammatic illustration of a number of a different tagswhich can be fixed to items, with the tags including a diode and a SAWdevice having a characteristic impulse response to carry the particularcode for the tag;

FIG. 7 is a diagrammatic illustration of the display of FIG. 5 in whichthe detected item is identified and displayed both in terms of itsdirection and range, with indicia on the display indicating to “go left”or “go right” relative to the centerline of the display;

FIG. 8 is a diagrammatic illustration of a SAW ID tag-specifictransmitter-tag-receiver configuration in which an interrogating signalcarrying a BPSK code illuminates a tag which carries a SAW correlator,with the received pulses when correlated having the indicated waveformand with receiver processing the correlator output to provide anindication of the found tag;

FIG. 9 is a diagrammatic illustration of a SAW ID tag generaltransmitter-tag-receiver configuration in which an oscillator is gatedto provide a general interrogation pulse and in which the interrogationpulse is detected by the tag, processed by the SAW correlator andbackscattered towards the receiver, with signal processing at thelocator detecting the presence of the coded tag and its identity;

FIG. 10 is a diagrammatic illustration of parallel LC tuning for a diodetag coupled to a bi-cone antenna;

FIG. 11 is a block diagram of a tag circuit including a dipole coupledto a low-pass filter in turn coupled to an amplifier and a non-linearcircuit such as a diode followed by a switch and a high-pass filter toproduce a harmonic backscattered signal;

FIG. 12 is a schematic diagram of a rectenna-type diode circuit utilizedto multiply the RF voltage from the interrogation signal to powerfollow-on smart chip technology in a tag;

FIG. 13 is a block diagram illustrating the utilization of a transmitterin which pseudo noise sequence modulation is used, the output of thetransmitter being delayed and provided to a demodulator, with the outputof the transmitter being frequency doubled and coupled to a mixer towhich the output of the demodulator is coupled, and with the mixedoutput applied to receiver stages;

FIG. 14 is a schematic diagram illustrating the utilization of a biasingbattery which extends the range of a tag employing a diode; and,

FIG. 15 is a block diagram of a regenerative tag that uses abattery-biased rectenna diode rectifier circuit for scavenging energyfrom an interrogation beam to provide a 10× range increase for the tag.

DETAILED DESCRIPTION

Central to the idea of the subject invention is the provision of alocator that is programmable to search for and locate specific items inan urban environment. As is often the case an individual has a need tolocate more than one type of item in his house or building; and whileitems can carry tags for identification purposes, an individual usuallyonly wants to locate one item at a time.

Tagged items may be spread around a room or building and it is thepurpose of the subject locator to be able to project a narrow beam tosweep out an area and to detect only the selected tagged item.

Referring now to FIG. 1, a locator 10 is a handheld device held by anindividual 12 which provides a narrow interrogation beam 14 that may beswept about an area in which a number of tagged items may exist.

In the illustrated embodiment Item 1 is a book, here illustrated at 16,which is buried in a couch 18 potentially covered up by a pillow 20.

The item carries an RFID tag 22 which in the preferred embodiment has asurface acoustic wave or SAW coding device. The coding device isprovided with an ID code by the construction of the fingers of the SAWdevice. It is contemplated in one embodiment that a locator kit willinclude as many as for instance 10 RFID tags, each with SAW devicesconfigured to provide different BPSK codes.

When the device is interrogated by beam 14, and as will be discussed,energy in the beam powers the RFID tag that in one embodiment carries adiode which after having been powered produces a harmonic backscattersignal at two times the frequency of the interrogating signal.

Thus, as illustrated at 23, the frequency of the interrogating energy isF and the backscattered and modulated return 24 is at twice thisinterrogating signal frequency.

As will be discussed, the identity of the detected tag is shown at adisplay 30, which, as can be seen in FIG. 2, includes graphics 32indicating the location of the item and the direction of the beam aswell as range from the locator to the item, with the range beingprovided by range rings 34.

Central to the ability of the locator to search for a particular tag isthe utilization of a keyboard 36 which is used to type in the name ofthe item to be located.

Referring back to FIG. 1, all of the tags in the kit are registered in acomputer 40 along with a name assigned to the particular tag by theindividual seeking to use the locator.

Thus, for instance, Item 1 can be tagged with the name “book”, Item 2with the name “magazine”, Item 3 with the name “drill”, and Item 4 withthe name “bolt”.

Locator 10 is provided with internal electronics that has a memory andCPU that can be synchronized with computer 40 and more importantly isable to upload tag codes of the registered items and item names fromcomputer 40.

In operation, individual 12 keys in the name of the item to be locatedwhich has associated with it a predetermined RFID tag code. In oneembodiment, the code is modulated onto the interrogating signal so thatonly the tag having the corresponding code is activated to providebackscatter radiation.

In this matter the coded signal from locator 10 is correlated with thecode in the RFID tag such that only the tag having the RFID coderadiates back towards the locator.

This correlation between the coded signal transmitted out and the codedsignal coming back to the locator increases the robustness of the systemand also increases range as will be discussed.

When a series of RFID tags is provided in a kit along with the locator,the individual using the locator registers item tags along with the tagname.

Referring back to FIG. 2, once the operator has keyed in the name of thesought-after item, he presses a FIND button 42. In the illustratedexample the named keyed in is “book”, and when the tagged book is found,the label for the item found is displayed as illustrated at 44. In thiscase the item found is the “book” in question.

The location of the item found is also displayed on display 30 by anicon 46 which appears when the backscatter return is received at thelocator. The position of the icon on display 30 within beam 32 indicatesthe direction of the sought-after item relative to the center line ofthe beam as well as its location. The position of the detected item isdisplayed relative to range rings 34 which indicate the distance of thedetected item from the locator.

Again referring back to FIG. 1, it is the purpose of the subjectinvention to find or detect specific tagged items within an urbanenvironment. The urban environment, unlike finding items in the open, isreplete with metal objects which cause multi-path distortions and alsodisturb the standing waves either transmitted towards the object havingthe tag or harmonically transmitted back from the tag.

It is a feature of the subject invention that various techniques areutilized to extend the range and thus the robustness of the locator whenspecifically used in an urban environment, in which the signals from thelocator may for instance need to pass through a wall 50 which attenuatessignals both in the forward and reverse directions, through floors, andthrough furniture. Note that the objects through which the interrogationbeam and the backscatter pass may both attenuate signals due to thematerial that it is made of and may also generate ghosts or reflectionsdue to metal in the various objects.

As will be discussed, it is important to have as much range as possiblefor the locator. Range in general is proportional to the power of thetransmitter within the locator which as mentioned before is limited dueto Federal Communications Commission requirements. In one embodiment,the output power of the locator may be varied to take into accountexisting or new FCC regulations.

Thus, an item locator that can be used in the open where there is nointerference from urban dwellings and structures or the like, when usedin an urban environment is severely range-limited.

It is therefore important to be able, within FCC regulations, to providerobust item location within buildings, through floors, behind walls,regardless of multi-path distortions. In one embodiment a batterylesstag location system provides cross ambiguity function processing usingcross correlation and frequency estimation techniques to extend therange 60 from individual 12 to the item in question.

Also, providing a range algorithm which works in urban environments isimportant if the range to the item is to be appropriately displayed ondisplay 30.

Rather than using amplitude alone, sophisticated range algorithms areemployed which operate, in one embodiment, with pseudo random noisecoding and frequency sweeping to be able to ascertain the distance ofthe tag from the locator. The cross ambiguity function, crosscorrelation, frequency estimation and range algorithms are describedhereinafter.

The ranging algorithm in one embodiment involves some combination ofveneer processing the cross ambiguity function peak to exploit the SNRmargin to get sub range bin resolution on the correlation. Thistechnique is well known in the art and can be implemented by fastfourier transform interpolation of the correlation peak. Alternativelyor additionally, the reader can fire a second waveform, namely awideband pulse that resolves the range to the object using range gatingtechniques. The correlation method is preferred as it is lesssusceptible to spurious reflections since the returned signal is markedwith the searched object's identification code.

Specifically, in one embodiment range determination involves the crossambiguity function as described in the literature, notably by Harry L.Van Trees, in “Detection, Estimation, and Modulation Theory, Part III”.The cross ambiguity function is a technique for jointly estimating rangeand Doppler frequency. This is done by essentially measuring thesimilarity between a complex-valued signal and a replica of it thatshifted in both time and frequency. If s_(T)(t) is the envelope of thetransmitted signal (i.e., the PN code) , and s_(R)(t) is the receivedsignal after frequency-down conversion, the resulting 2-D time-frequencycross ambiguity function is given by

Φ(τ,f)=|∫_(−∞) ^(∞) s _(R)(t−τ/2)s* _(T)(t+τ/2)e ^(j2πft) dt| ²

Here, the estimated time-delay and Doppler-frequency difference betweenthe two signals are determined by the location of the peak of thisfunction over τ and f. The range is then simply calculated from thetime-delay. Drift in the transmit oscillator can be estimated by firstdividing the received signal data into subsections, and then computingthe time-frequency cross ambiguity function for each section (andtracking the frequency). In addition, the estimated frequency from theambiguity function can be used to adjust the transmit oscillator suchthat the estimated frequency drift is driven to zero, thereby possiblymaking the computation of subsequent ambiguity functions easier.

In summary, there are two approaches to estimating the range to the tag.The first approach uses correlation processing. When the object is faraway, there is little excess SNR and one uses relative amplitudes vs.pointing direction to get a bearing to the object. As one approaches theobject from a constant bearing, the amplitude continues to grow untilthere is sufficient SNR to begin interpolating the correlation peak.

This interpolation is necessary because one anticipates interrogationwaveform bandwidths of between 1 and 20 MHz, corresponding to range binsgoing between 1000 feet and 50 feet respectively. However, byinterpolating within the range bin having peak range, the correlationpeak location can be estimated to a small fraction of the width of therange bin. This corresponds to ranges of between 100 feet down to ½foot, depending on the interrogation bandwidth and the SNR. Thistechnique is well known from radar range estimation and passivecommunications emitter ranging and navigation.

An especially efficient implementation involves zero-padded FFTprocessing as is described in many references including for example,Seymour Stein, “Algorithms for Ambiguity Function Processing”, IEEETransactions on Acoustics, Speech and Signal Processing volume ASSP-29,June 1981 pp 588-599. Due to advances in microelectronics, thesesophisticated signal processing techniques may now be performed in acomparatively small and inexpensive processor suitable for consumerproducts.

In addition, note that very few range bins are actually necessary due tothe indoor geometries anticipated. In order to correct for oscillatorfrequency drift in the transmitter of the locator, frequency estimationtechniques are utilized to detect from the returned radiation any shiftin the initial oscillator output. The frequency of the returned signalmay be tested for the cross ambiguity function and the results are usedto adjust the oscillator in the locator. Moreover, few frequencies aretested for the cross ambiguity function as the frequency ranges correctsmall local oscillator drifts during the interrogation process only.

A second more simplified ranging approach is to transmit a very widebandranging pulse. When the pulse rise time is only a few nanoseconds,measuring the return pulse time delay will provide range resolution to afew feet. However, this approach suffers from spurious reflections bymetallic objects. Thus, the returned pulse must have sufficientamplitude to detect the code transmitted by the tag as part ofdemodulating the received signal.

Moreover, as part of the subject invention, it has been found thatrather than using a batteryless tag, providing a small thin film batteryto bias the diode in the tag increases the range by 50%.

It has been found that pre-biasing the diode increases its sensitivityas a detector and further increases the backscatter power so that tagscan be for instance detected in terms of miles not feet.

Moreover, the subject system can be used in urban environments to locateindividuals which who carry tags, such as for instance foridentification of friend or foe, in which the individuals can be locatedbehind buildings or walls. The ability to detect tagged individuals inan urban environment is in part due to the increased range provided bydiode biasing.

Thus, for people location as well as item location, the subjecttechnique is made robust through the utilization of diode biasing and itis the finding of the subject invention that over batteryless tags, a50% increase in range is achievable as illustrated by arrow 62.

Referring now to FIG. 3, locator 10 houses a transmitter 70 coupled viaa circulator 72 to a directional antenna 74, which may be a yagi orhelix antenna that projects a narrow interrogating beam 76 towards atagged item 78 having a tag 80. Tag 80 is diagrammatically shown ashaving an input antenna 82 coupled to a diode 84, with an output antenna86 coupled to the output of the diode.

Modulation 88 is applied to modulate the signal from diode 84 such thatthe backscatter radiation 90 is coded, with the code being the uniquecode associated with the item.

Locator 10 has a receiver 92 which is coupled to a processor 94 that hasembedded in it the cross ambiguity function and ranging algorithmsdescribed above, with the item ID code and name inputted to processor 94from the aforementioned hot-synch operation.

The output of the processor includes the range of the tagged item aswell as its ID or name. As will be appreciated, the name of the detecteditem is coupled to display 30.

In one embodiment processor 94 outputs signals which not only relate tothe ranging information derivable from the received signal, but also tothe direction of the item to permit generating instructions like “moveleft” as illustrated at 96 or “move right” which is illustrated at 98.Thus, the individual using locator 10 knows which way to swing the beamof the device. The direction of the item can be derived from monopulsetechnology which is used to detect the direction of the tag. Inmonopulse processing the received pulse is received by two receiveantennas spaced apart by a wavelength. The phase difference between thetwo received signals is then used to detect the direction of the source,namely the tag. This system is effective to indicate whether the tag isto the “left” or the “right” of the locator centerline, or to provide anarrow on the display which points towards the tag.

The signal-to-noise ratio may be a factor in the ability to utilize themonopulse technique; and multi-path and standing waves, especially in aroom with very reflective walls, as in commercial buildings, mayadversely affect the monopulse determination.

As will be appreciated, higher frequencies are more useful in tagdetection because the interrogator can pick the first returned signaland ignore later ones. Thus for monopulse systems, operating at 5gigahertz is preferred. Also the separation of the two receive antennasis only 1 inch at this frequency, making the handheld applicationachievable.

With the respect to ranging, utilizing 5 nanosecond chirped pulsesresults in a 1 foot resolution. Moreover, if the range resolution isgiven by 1/BW and the bandwidth is 1 gigahertz one obtains a 1 footresolution.

Referring back to FIG. 3, from the output of processor 94 comes the nameof the detected item, in this case the name “magazine” as shown at 98.Optionally an alarm unit 100 may be coupled to processor 94 to providean audio alarm that changes either in intensity or frequency when thelocator closes in on a tagged item.

Processor 94 can also be used to output a PN code 95 to transmitter 70for the encoding of the interrogation beam from directional antenna 74to a tag coded with the PN code. This improves the robustness of thesubject system by increasing the signal-to-noise ratio and thus therange. It also makes the system more immune to noise and other signals.In one embodiment, the PN code is uploaded to processor 40 from computer40 of FIG. 1.

Thus, while the subject system may be utilized with an unencodedinterrogation signal from transmitter 70, in FIG. 4 the output oftransmitter 70 is modulated with a code corresponding to the code of thetag sought. If the tag is encoded using a SAW device, then the SAWdevice code is that which is modulated onto the signal from transmitter70. Note that in this embodiment, a SAW device 102 is employed whichcarries its own unique individual code in terms of its impulse response.Here, the SAW device is coupled between diode 84 and antenna 86.

In operation, the interrogating signal from transmitter 70 is detectedby diode 84 and the output signal is modulated by the SAW device 102impulse characteristic. This occurs only when the signal detected atdiode 84 contains the same SAW device code.

Referring now to FIG. 5, locator device 10 may be in the form of aflashlight or tubular configuration having keyboard 36 thereon, withdisplay 30 providing the information described hereinbefore.

In order to fit in a flashlight type device the directional antenna maybe a miniature yagi or helix, assuming the system operates close to 6gigahertz, so as to be able to provide a reasonable sized directionalantenna.

As shown at FIG. 6 individual tags 80 may be provided with their own SAWdevices 102 to provide different impulse responses and thereforedifferent codes to the tag.

As illustrated in FIG. 7 in greater detail, display 30 shows therepresentation of beam 32, range rings 34, item 46 and instructions asto which way to move the locator beam, namely “go left” 96 and “goright” 98. Also the identity of the found item is illustrated at 44.

Referring now to FIG. 8 one of the more attractive ways to increase thegain of the system and to make it more robust is to provide a codedinterrogation beam in which the coded beam corresponds in code to thecode of the SAW delay line in the tag.

Here, a diodeless tag can be provided having two antennas and a SAWdelay line correlator in which the SAW delay line avoids clutter byusing a time delay.

As can be seen, a transmitter can include a BPSK coded oscillator 120which switches a pulse at 122 to an antenna 124. Here the interrogatingsignal 126 includes a coded interrogation pulse 128.

As illustrated, tag 130 includes a receive antenna 132 and a transmitantenna 134, with the receive antenna and transmit antenna being dipolesin one embodiment. Here, the input to a SAW device 136 is at one end ofthe SAW delay line correlator.

The impulse response of the SAW device is determined by itsinterdigitated fingers 138, such that as the acoustic wave 140propagates down the correlator, the impulse response of the SAW deviceimparts a code to the acoustic wave.

The acoustic wave contains the original signal at F₁ modulated by theoutput of the acoustic wave delay line. This modulated signal istransmitted out of dipole 134. At the locator the received pulses 142with modulated code are correlated by a correlator 144, the output ofwhich is detected by a signal processing detector 146 coupled to antenna148.

Note that the so called “barcode” information is contained in the SAWdevice dispersion due to specifically placed interdigitated reflectorsin the SAW path, corresponding to one or zero as specific delays alongthe path. Note also that the use of SAW devices requires a hard wiredapproach in which the coding of the SAW devices is provided at the timeof tag manufacture.

It is a design goal for the subject system to have a range in terms of afew miles and to have a directionality for the interrogation beam of forinstance less than 20° for its beam width. In a preferred embodiment ofthe subject invention, backscattering is preferred over remodulationbecause backscattering reradiates 100% of the captured energy.Additionally, pseudo noise, PN or other modulation produces a 30 dBprocessing gain, whereas frequency dispersion provides additionaldiscrimination.

Backscattering using SAW delay lines and BPSK Barker coding involves theidea of passively coding and decoding a transmission. Only the tag withthe correct decoder or encoder on the tag, namely a SAW correlator, willgive a positive response at the interrogator receiver.

Assuming a 1 megahertz bandwidth chosen because the tag can modulate itsretransmission, the receiver has a correlator which converts themodulated signal to a detected pulse. Alternatively, the interrogatortransmit signal can be modulated and the tag SAW acts as a correlator,with the output from the tag being a pulsed unmodulated CW signal.

No matter what assumptions are made about bandwidth broadening andprocessing gain from the correlator, these two terms are the inverse ofeach other, at least ideally. Hence, a bandwidth of 1 megahertz and aprocessing gain of 30 dB come down to an equivalent of 1 kilohertzbandwidth at the receiver for a CW signal.

Referring to FIG. 9, one can have a general interrogation pulse 150produced by a gated oscillator 152 switched by a switch 154 to antenna156 from whence it propagates towards tag 160. This tag is identical totag 130, with like elements bearing like reference characters.

The signal transmitted back to the interrogator is the interrogationpulse modulated by the tag code as illustrated at 162. This signal isincident on antenna 148, is amplified at 164 and is then provided tosignal processing detector 146

The use of the SAW delay device and correlator provides a uniqueidentity to the tag that can be detected assuming the codedinterrogation pulse returned from the tag is amplified.

As to tuning antennas to a diode, and referring now to FIG. 10, aParallel LC tuning circuit tunes a bi-cone antenna having sections 172and 174 to a diode 176.

The tuning for the diode includes a 22 nanohenry inductor 178 inparallel with a 0.4 pF capacitor 180, with a 12 nanohenry inductor 182between the diode antenna section 172 and 12 nanohenry inductor 184coupled between the two bi-cone halves.

Note, the series LC circuit in parallel with the diode tunes theharmonic and inductance in parallel with the diode to the fundamentalfrequency using the capacitance of the diode at achieved resonance. Theparallel inductance also prohibits reverse dc voltage buildup whichwould degrade harmonic response.

It will be appreciated that tuned circuits acting as filters at thereceive antenna for the RFID tag can code the tag as to frequency.Backscattered radiation from a tag utilizing these tuned circuits canthus be distinguished from other backscatter by filtering at thelocator's receiver.

Referring now to FIG. 11, in one embodiment, the tag includes a dipoleantenna 190 coupled to a low pass filter 192 set to the frequency of theincoming wave, namely F₁. The output of the filter is amplified at 194and is coupled to a non-linear circuit 196, which can include a diode198.

The output of the non-linear circuit is applied to switch 200 which isused to stop self oscillation. The output of switch 200 is applied to ahigh-pass filter 202 set to pass frequencies above 2 F₁, with the outputof this filter applied to dipole antenna 204.

With a 10 watt transmit signal, the circuit shown in FIG. 11 would havea 1 kilometer range assuming a 40 dB gain at amplifier 194. Assumingthat one loses 3 dB across the switch because of energy in side lobes,switch 200 in one embodiment provides a greater than 40 dB isolationbetween the two antennas at all frequencies to stop self oscillation.

In one embodiment the input dipole is tuned to 975 megahertz, whereasthe output dipole is set to be resonant at 1950 megahertz.

While the foregoing has described simplified tags either employing a SAWdevice or a non-linear element, namely a diode, it is possible toprovide tags with a so-called smart chip. It is the purpose of the smartchip to code the tag with its unique identity and for instance, toprovide other information that can be modulated onto the signal returnedfrom the tag.

However, powering of smart chips with the energy from the interrogationbeam is problematical. For instance, typically the RF voltage at a tagdipole is on the order of 0.1 RF volts. However, by using voltagemultiplication circuits one can multiply the RF voltage by a factor of10 and output 1 volt dc. To do so, one uses a rectenna charging circuitsuch as shown in FIG. 12.

Referring now to FIG. 12, a rectenna charging circuit 120 takes the RFinput voltage at 122, rectifies it and multiplies it by using a numberof diodes and capacitors. Thus diodes 124, 126, 128, 130, 132 and 134are utilized in a rectification and voltage multiplication scheme inwhich energy is stored on capacitors 136, 138, 140, 142, 144 and 146.Note that the output capacitor is a 1 nanofarad capacitor, whereas theother capacitors are 20 picofarads.

The result is a voltage at output 150 which is sufficiently high topower smart tag circuits. As will be discussed, battery biasing therectenna charging circuit results in better voltage multiplication. Thebatteries used for such biasing are long lived watch batteries thatprovide a 1 microamp bias for in excess of 10 years.

If one were to attempt to place batteries on tags to power the smartchips, the batteries would run down in a matter of hours. It istherefore important to use rectenna technology when using smart chips.Moreover, if one is using a smart chip with a battery, the rectennacircuits can be used to sense an interrogation beam and turn on thebattery connection to the smart chip, thus to save on battery drain.

Powering smart chips directly from a rectenna circuit eliminates theneed for long-lived large batteries. By using battery diode biasing onecan improve the voltage multiplication of rectenna circuits to the pointwhere the rectenna circuit can power smart chips without having to uselarge short-lived batteries.

From the point of view of remodulation, in a typical RFID tag, arectenna circuit is coupled to the receive antenna of the tag. Theoutput of the rectenna circuit is typically a 1 volt signal that is usedto power an oscillator in the tag. In one embodiment, the oscillatorincludes a PIN diode which is turned on and off in accordance with theID code for the tag. It is preferable that this oscillator output afrequency which is different from that of the interrogating beam. In oneembodiment, the frequency output by the tag is twice the fundamentalfrequency of the interrogating beam. The modulated output of theoscillator is then coupled to the transmit antenna of the tag.

The result is that the backscattered radiation from the tag has a codeimpressed upon it which distinguishes it from backscatter fromelectronic equipment in the scanned area. Thus the locator candistinguish between the modulated backscatter from the tag andbackscatter from radios and other equipment which backscatter isunmodulated. Thus, in a typical RFID tag system, a remodulationtechnique is utilized to transmit information from the tag to thelocator.

In another remodulation technique, one may seek to remodulatebackscatter with a frequency offset and this can be done using phaseshifters and phase shifts/Doppler shift modulation to offset the carrierfrequency.

The idea in one embodiment is to 100% convert the interrogator signal atthe tag to a different frequency using phase modulation. Because theconversion between to the two frequencies is ideally 100%, the tagprovides low loss and the new frequency does not have background signalissues at the receiver.

Referring to FIG. 13, another RFID technique involves phase shifting theoutput of the transmitter at the locator in accordance with a PN code.To implement this technique, a transmitter 220 in a locator is coupledto a 0/90° modulator 222 which is coupled to an antenna 224 thattransmits at the fundamental frequency. A pseudo noise sequencegenerated at 226 is applied to modulator 220 to phase shift thetransmitted signal. The PN sequence is also coupled to a delay line 228which is applied to a 0/180° demodulator 230 to which a receive antenna232 is coupled. The receive antenna receives the harmonic response ofthe tag. The tag backscatters a signal at 2 F modulated with a PN codesequence that is 0°/180° modulated with this code.

For baseband processing, the locator frequency doubler 234 doubles thetransmit frequency and applies the doubled frequency to a mixer 236which mixes the output of the 0/180° demodulator receiving the 2 F₁signal to baseband. The output of the mixer is then coupled to receiverstages as illustrated by arrow 238. This describes a correlationtechnique in which the transmitted pseudo noise sequence is delayed andapplied to the 0/180° demodulator 230 to correlate the 2 F₁ tag signalwith the encoded transmitted signal.

Referring now to FIG. 14, a biased diode tag circuit is shown in which abiasing battery 250 is utilized to bias a diode 252. Here it can be seenthat a 100 ohm resistor 254 is connected from the positive terminal ofbattery 250 through a 20 nanohenry inductor 254 to diode 252, whereasthe negative terminal battery 250 is connected through a 200 nanohenryinductor 256 to the other side of diode 252. Here, the inductors serveas RF chokes.

The output of the diode is coupled through a capacitor 260 and through atuning circuit comprised of inductors 262, 264, 266, and 268 thatcouples the diode out to the secondary of a transformer 270 used tomeasure the harmonic performance of the diode.

Note that there is an output capacitor 272 and an LC circuit comprisinginductor 262 and a further capacitor 274.

The circuit shown in FIG. 14 was used to establish the rangeaugmentation of the system when utilizing a battery-biased diode. Therange augmentation was tested by using a 4:1 Transformer comprised ofinductors 268 and 270. It was found that there was a 10 dB increase inpower resulted when battery biasing the diode. This translates into a10× output power increase in the harmonic output of the diode thattranslates into a 50% range increase. Put another way, with batterybiasing at least 10 dB lower minimum detectable signal was achieved with1/R² propogation loss as a standard of comparison. This could result ina 3.16:1 increase in range for 1/10th the power for the same outputvoltage. If one uses an inverse fourth power propogation loss, oneobtains a 1.77:1 or a 77% increase in range.

A 10 dB increase in harmonic response of the diodes means that theincident power can be 5 dB lower, and the same harmonic power will begenerated. This is due to the Power² dependence of the harmonic power onthe fundamental frequency power. Hence the range is (R2/R1)2=10̂(5dB/10)=>R2=1.77*R1. This is a 77% increase in range.

In summary, as to diode biasing it has been found that biasing a diodeimproves the harmonic response of the diode, and therefore increases theeffective range of the system. Secondly, when voltage multiplicationtechniques using a series of diodes is employed, the effectiveness ofthe voltage multiplication is improved with diode biasing.

Thus the subject invention can either be diodeless, having a diode whichoperates harmonically, or can involve the use of a smart chip powered bythe voltage multiplication afforded by a rectenna type circuit.

While battery biasing of diodes has been described as resulting in a 50%range improvement over unbiased diodes, when battery biasing is usedwith rectenna circuits and the rectenna circuit is to power a so-calledregenerative tag, then a 10× range improvement results.

In a regenerative tag the signal from the rectenna circuit is appliedacross a capacitor which when discharged provides a signal that ismodulated with the ID of the tag. This signal is transmitted from thetag using the transmit antenna for the tag. Thus, while the 50% increaseapplies to passive tags involving diodes, when a regenerative tag uses arectenna circuit augmented by battery biasing, the range increase ismuch better than 50% and approaches 10 times that associated withunbiased diodes.

More particularly, note that RF power scavenging systems allow for thedesign of batteryless radio-devices. The systems convert RF power intoDC power in which electronic circuits can operate. DC power may becontinuous or pulsed. If micro-watt levels of power are available,scavenging systems such as rectenna circuits may be biased to greatlyimprove performance, on the order of 15-20 dB.

Since regenerative tags are powered by rectenna circuits, the powerreceived by a regenerative tag being proportional to the inverse squareof the range results in a 20 dB improvement in power scavenging; andthis produces a ten-fold increase in range.

It could be thought that if one had a battery already why would one wantto be rectifying RF from an interrogation beam to make DC? The reason isthat such batteries would have an exceedingly short life.

However when battery biasing rectennas with a small nanocurrent watchbattery that biases the diodes, one achieves a 10+ year tag life with ahundred-fold improvement in power scavenging efficiency. Improvingscavenging efficiency is why one gets 1/r² range dependence and aten-fold range increase. This is to be distinguished from putting abattery-powered receiver in a tag which would be expensive andinconvenient for a consumer due to limited battery life and the addedcost.

In the subject invention, in one embodiment one has a nanocurrentbattery to bias the rectenna circuit so one can have a battery poweredreceiver in the tag pretty much for free without any decrease in othercapabilities or convenience.

In a further embodiment, when using a large battery to power theregenerative tag, the tag is dormant and in a batteryless mode until itgets addressed. Then in a wake up function its large battery turns on,enabling many other functions. Note, if the large battery powers areceiver, it will burn out quickly absent the above wake up function.However, large batteries and the wake up function can be avoided if onebiases a diode network with a small battery, since it will last foryears.

Thus nanocurrent biasing dramatically improves power scavenging anddramatically improves tag range.

Referring now to FIG. 15, a regenerative tag 300 is shown to include areceive antenna 302, and a rectenna diode rectifier circuit 304 coupledto an energy storage element 308. The output from rectenna circuit 304ranges between 50-400 millivolts which is stored on a 1-10 μF capacitor.

The output of energy storage element 306 is coupled to a hystereticswitch 308 which outputs 50-200 millivolts based on the output of ananowatt comparator 310. This voltage is used to power transmitter 312that is coupled to transmit antenna 314, which in one embodiment has a−26 dBm characteristic at 2 ghz.

In one embodiment, transmitter 312 is an oscillator that includes a LCcircuit coupled to a tunnel diode oscillator which puts out 50-200 μWand which draws at approximately 65 millivolts at 1 milliamp.

As can be seen at 316, a tag-carried battery is coupled to rectennacircuit 304 to bias the diodes thereof with a nanoamp biasing voltagesuch as can be provided by a typical watch battery, or by lithiumhydride thin film technology.

Hysteretic switch 308 is triggered on when the energy in energy storageelement 306 exceeds a predetermined threshold such that transmitter 312can be operated once the scavenged energy is sufficient for operation.When the hysteretic switch discharges, the transmitter emits the tagidentification waveform. The computer contained in the tag readercontains a stored replica of this waveform. This replica is used for thecorrelation processing/cross ambiguity function processing methodpreviously described. Ideally, the stored waveform is a baseband digitalrecording of a tag transmission created during tag initialization.Modern memory devices provide for several gigabytes of storage atinexpensive prices, making this feasible even for long-duration, widebandwidth waveforms. Even when memory is limited, it is possible toreconstruct a long-time recording from a short sequence by remodulatingthe short sequence with the RFID code transmitted by the tag or bysimply concatenating the unique segments of the tag waveform thatconstitute the periodic tag transmission. Through these means, thecorrelation processing algorithm obtains the processing gains necessaryto detect the tag while minimizing the energy radiated by the tag, alimited quantity due to the need to remotely charge the tag usingelectromagnetic radiation from the interrogator.

It has been found that battery biasing the rectenna circuit improves theefficiency of the diode circuits in the rectenna to such an extent thatthe range of the regenerative tag is increased 10× over that associatedwith an unbiased rectenna circuit. This is because the scavengingability of the multiple diode rectifier is significantly increased overan unbiased rectenna circuit.

It is therefore a finding of this invention that battery biasing arectenna circuit results in both increased efficiency for the circuitand an order of magnitude increase in range for the tag.

It should be noted that the interrogator charges the tag through theinput rectifier consisting of a lattice of diodes and capacitors. Whenthe capacitor crosses a threshold voltage, the hysteretic switchdischarges the capacitor into the oscillator circuit, therebytransmitting a signal. The energy stored into the tag in a given amountof time is inversely proportional to the square of the range to the tag.

In many cases, it is much easier to detect a tag emitting a waveformwith total energy E0 than it is to charge a tag with the same amount ofenergy E0. Therefore, most of the engineering and design goes intofiguring out efficient ways to charge the tag, because it is known thatthe range necessary to deposit energy E0 into the tag in a short periodtime is typically far shorter than the detectable range of the waveform.The performance of the tag depends on the amount of energy that can becharged, a quantity that is inversely proportional to the range to thetag.

Biasing the diodes in the rectifier circuit with a small voltage and atiny amount of leakage current increases the rectification efficiency bya factor of 100. That is, one hundred times as much energy per time isdeposited in the capacitor for a given power presented at the rectifierinput. Since the efficiency is one hundred times greater and the inputpower is inversely proportional to the range squared, the effect is thesame as increasing the range from the interrogator by a factor of ten.

In either case, the minimum detection range is far longer than theminimum tag charging range, so the minimum operating range goes up by afactor ten.

While the present invention has been described in connection with thepreferred embodiments of the various figures, it is to be understoodthat other similar embodiments may be used or modifications or additionsmay be made to the described embodiment for performing the same functionof the present invention without deviating therefrom. Therefore, thepresent invention should not be limited to any single embodiment, butrather construed in breadth and scope in accordance with the recitationof the appended claims.

1. A method for extending the range of an RFID system which employs atransmitter for transmitting an interrogation beam towards an RFID tag,the RFID tag using a non-linear element including a diode for extractingthe energy from the interrogation beam and backscatters a coded signal,comprising the step of biasing the diode with a tag-carried battery,whereby the range of the RFID system is improved over an unbiased diode.2. The method of claim 1, wherein the range improvement through the useof battery biasing the diode is at least 50% over an unbiased diode. 3.The method of claim 1, wherein the RFID system includes a transmitantenna and the tag, the tag having a SAW device that has an impulsecharacteristic which codes the output of the tag, and wherein the SAWdevice is coupled between the diode and the transmit antenna for thetag.
 4. The method of claim 1, wherein the diode biasing improves theharmonic response of the diode.
 5. The method of claim 1, wherein thestep of battery biasing the diode includes utilizing a lithium ionbattery.
 6. The method of claim 5, wherein the battery is a thin filmbattery.
 7. The method of claim 1, wherein the battery includes alithium battery having silicon nano wire anodes.
 8. The method of claim1, further comprising increasing the signal-to-noise ratio by frequencysweeping in conjunction with pseudo noise code.
 9. The method of claim1, further comprising correcting for frequency and range with a crossambiguity function.
 10. The method of claim 9, wherein the crossambiguity function is given by: Φ(τ, f)=|∫_(−∞)^(∞)s_(R)(t−τ/2)s*_(T)(t+τ/2)e^(j2πft)dt|² wherein s.sub.T(t) is theenvelope of the transmitted signal, and s.sub.R(t) is a received signalafter frequency-down conversion.
 11. The method of claim 1, furthercomprising correcting for oscillator shift using a frequency estimationsystem.
 12. The method of claim 9, further comprising estimating theproximity of an article using a ranging algorithm.
 13. The method ofclaim 1, wherein the transmitter employs a highly directional antenna.14. The method of claim 1, further comprising correlating a receivedsignal with an anticipated signal in order to estimate signal amplitude.